I have had fun recently using a piece of A4 paper and just doodling over it with mandalas and other patterns to fill the space...
Here's a free copy of one such doodle which you can download and colour to your heart's content.
(As the artist I retain the copyright of my designs. You can print as many times as you like for your own enjoyment but please do not distribute or duplicate without my permission.)
More of these doodles are available to buy in my Etsy shop.